Manually moving excavator remove their current cargo

Issue description

When manually moving and excavator that already have a cargo, the cargo disappear.

Reproduction steps

1. Have and excavator with some cargo.
2. Manually move the excavator using the "target" button in the excavator UI.
3. The excavator should dig a new cargo.

Game version and build number

v0.7.1a (b413)

Operating system

Windows

The cargo is moved to a storage or a shipyard.

It is done on purpose as managing shovel state whilst excavators are driving around would be too complex.